The most traditional and consolidated process for the forming of products based on compact thermoplastic material is injection moulding; this is the typical technological method for the manufacture of articles made of thermoplastic material in which material in liquid state is placed in a cavity of a permanent die (mould), driven by an “injection” force”.
The injection moulding includes, in short, a “plasticizing and injection group”, in which the polymer is melted, and a die into which molten polymer is injected, having a chamber that gives shape to the product. The apparatus uses plastic granules as basic raw material; the granules are made to pass inside a cylinder by means of a worm screw (Archimedes screw). The process of fusion inside the cylinder is achieved by means of heat induced by electrical resistances, and by friction generated by the movement of the Archimedes screw inside the cylinder.
The melting temperature (or glass transition in the case of amorphous materials) depends on the type of material that is used; normally it ranges from 160° C. for low density polyethylene (LDPE) up to 300° C. for polycarbonate (PC).
Once melted, the material is injected into the forming chamber through the action of the Archimedes screw. The injection pressure to which the forming cavity is subjected is usually of the order of 300-600 kg/cm2. The size of the presses and their energy consumption are strongly influenced by the surface of the product to be moulded, and the pressure applied during the injection step.
The die internally defines a closed forming chamber which determines the shape of the object to be obtained, enables the molten plastic to cool in a short time and ejects the finished solid product, by means of appropriate mechanical means such extractors.